The way to go, imo. Also depending on your speaker load you could change the opt's (if your diy inclined) I have a pair of SE84 cs that I changed the opt's to Trancender 3k into 8 ohm's. I now have more power than I know what to do with a better mod might have been to get 1.5k opt's and pull one of the sv83's from the amps but at least I'm getting window-shaking power in the bass from a pair of HDT's! and that is no mean feat.

The problem with using lower value primary OPT is that you will also raise distortion, so depending on your speakers and listening habits it won't always work.

The SE84 puts out about 2.5W. A properly designed EL34 SET could give you 5-6W, but in reality that's not a lot more power, about 3db. So do you really need more power?

An EL34 could be installed in a SE84 (after a socket change) but it would tax the existing power supply since it would need to draw about double the current of the SV83. Also, you would need to replace all the resistors in the PS to different values because the voltage drop would be excessive with the new EL34. Next, you'd probably need higher cap values to give you the same ripple reduction of the original PS. Furthermore, the OPT should be changed to 5K to 6ohm or so to load the EL34 properly. Finally, the bias resistor would need to be changed to a different value and probably a higher wattage rating to bias the EL34 tubes properly